Brewer began her television acting career in 2003 with the role of Allana in the first series of The Sleepover Club. The part lasted only 11 episodes but saw her playing alongside other future Neighbours stars Eliza Taylor-Cotter (Janae Timmins) and Caitlin Stasey (Rachel Kinski). In 2005, Ashleigh gained a second TV role in long running cop series Blue Healers; she played Lelah Burton in the episode "Face Value". Her next television part came the following year playing Gracie in one episode of H2O: Just Add Water. She later reprised this role in 2008 for a further three episodes.

After finishing school, Brewer decided to pursue her acting career by moving to Melbourne and searching for work. She gave herself just three weeks and in which time she auditioned to play the character Kate Ramsay in Neighbours and won the role. She made her debut appearance on 15th May 2009.

Ashleigh lists her favourite films as Gran Torino, Revolutionary Road, Beyond Borders, Elizabeth Town, Garden State, Blood Diamond and Girl Interrupted; Family Guy is one of her favourite TV shows; and her favourite music comes from The Fray, Bloc Party, Sigur Ros, Daft Punk and Jamie Cullum. She currently shares an apartment in Melbourne with fellow Queenslander Margot Robbie who plays Donna Freedman in Neighbours.